The present invention relates generally to lasers, and specifically to lasers generating pulses at high rates.
There is a continuing demand for increasing the rate of transference of data in data communication systems. Optical communication systems are able to satisfy the demand because of their inherently extremely high bandwidth, and one of the components of such a communication system is a source able to generate optical pulses at very high repetition rates. Semiconductor laser diodes operating at wavelengths of the order of 1 xcexcm form the basis of many sources known in the art.
An article titled xe2x80x9c5.5-mm Long InGaAsP Monolithic Extended-Cavity Laser with an Integrated Bragg-Reflector for Active Mode-Locking,xe2x80x9d by Hansen et al., in the March, 1992, issue of IEEE Photonics Technology Letters, describes a monolithic mode-locked, semiconductor laser which generates transform-limited 20 ps wide pulses of 1.55 xcexcm wavelength at a rate of 8.1 GHz.
An article titled xe2x80x9cMonolithic Colliding-Pulse Mode-Locked Quantum-Well Lasers,xe2x80x9d by Chen et al., in the October, 1992, issue of IEEE Journal of Quantum Electronics, describes a monolithic mode-locked semiconductor laser generating pulses at ultra high rates, up to 160 GHz. The use of colliding pulses at a saturable absorber incorporated in the monolithic cavity further shortens the pulses, so that pulses having widths of the order of 1 ps are produced.
A drawback common to all monolithic constructions, however, is that manufacturing process limitations cause inherently wide ranges in emitted wavelength and repetition rate. The drawback can be overcome by using an external cavity system, comprising a semiconductor laser chip and an external narrow band element, typically a fiber Bragg grating (FBG).
European Patent Application 949,729/A2, to Meliga et al., describes a module having a semiconductor laser chip coupled to an external grating written in a fiber optic. A portion of the fiber optic couples the chip and the grating. The grating acts as a partially reflecting mirror, emitting light having a wavelength defined by the grating spacing into the fiber optic.
U.S. Pat. No. 5,305,336 to Adar et al., whose disclosure is incorporated herein by reference, describes a semiconductor laser chip coupled to an external grating. A DC bias and a radio-frequency (RF) current drive the chip via two electrodes, one of which functions as a ground electrode. The RF current and DC bias modulate the gain of the chip, and switches it between a net gain mode and a net absorption mode, so that the system provides pulses at the radio-frequency. The radio-frequency is set close to a fundamental cavity frequency, defined by a time for pulses within the system to travel a round-trip, which has the effect of mode-locking the system and emitting light pulses.
It is an object of some aspects of the present invention to provide apparatus and methods for generating short coherent optical pulses having a high repetition rate.
It is a further object of some aspects of the present invention to provide apparatus and methods for setting the repetition rate of the optical pulses independent of a wavelength of the pulses.
In preferred embodiments of the present invention, an optical system couples a semiconductor diode laser device to a fiber optic comprising a wavelength selective partial reflector, most preferably a fiber Bragg grating (FBG). The diode laser device comprises a relatively long gain section, and a short section operating as an electrically modulated saturable absorber, each section being controlled by a separate electrode. The device preferably has a third ground electrode. One facet of the device is coated to act as a first highly reflecting mirror. The partial reflector acts as a second mirror, so that an optical resonant cavity is formed between the two mirrors. When the partial reflector comprises an FBG, the wavelength at which the cavity resonates, and which is partially transmitted via the FBG into the fiber optic, is defined by the grating period of the FBG. An optical length of the cavity can be set by positioning the optical system and/or the fiber optic relative to the laser device, thus enabling the cavity to be tuned to the wavelength defined by the partial reflector.
A radio-frequency (RF) signal with a DC bias is injected into the saturable absorber section. The period of the RF signal is set so that it corresponds to the time for a pulse to make a round-trip within the cavity, thus locking the modes of the laser in phase and causing the laser device to emit short mode-locked pulses at a repetition rate equal to the frequency of the RF signal. The saturable absorber section in the cavity is positioned to cause a colliding pulse effect in the saturable absorber, further shortening the pulses so that the temporal pulse widths are effectively at the transform limit set by the frequency bandwidth of the partial reflector. Thus, the combination of the dual-section laser device coupled to the wavelength selective partial reflector enables the laser cavity to be produced so as to generate short transform-limited pulses having a substantially invariant wavelength. Furthermore, the repetition rate of the pulses can be conveniently set independent of the wavelength by appropriately setting the length of the cavity.
In some preferred embodiments of the present invention, the saturable absorber section is positioned adjacent to the highly reflecting facet, so that the pulses propagating in the cavity collide at the facet. In other preferred embodiments of the present invention, the saturable absorber section is positioned at an optical center of the cavity, so that pulses reflected from the opposing cavity mirrors collide in the absorber section.
In some preferred embodiments of the present invention, the optical system coupling the output of the laser device to the fiber optic comprises a single converging lens separated from the device and the fiber optic. Positions of the lens and the fiber optic are independently set when adjusting the laser cavity. In an alternative preferred embodiment, the single lens is cemented to, or is integral with, the fiber optic, so that settings for the cavity are made by adjusting the position of the fiber optic. The single lens focuses the diverging output of the device onto the fiber optic.
In other preferred embodiments of the present invention, the optical system comprises a plurality of lenses, one of which may be in contact or integral with the fiber optic. As for the single lens, the plurality of lenses focus the diverging output of the device onto the fiber optic.
There is therefore provided, according to a preferred embodiment of the present invention, an optical pulse generator, including:
a semiconductor device, which includes:
an optically-active region including a gain section and a saturable absorber (SA) section, which are adapted to generate coherent optical pulses;
an output facet for coupling therethrough of the optical pulses generated in the optically-active region; and
an SA electrode for application of a radio-frequency (RF) modulation of a desired frequency to the SA section; and
an optical output assembly, optically coupled to the output facet of the semiconductor device so as to partially reflect the coherent optical pulses within a predetermined wavelength range, and positioned so as to form, together with the semiconductor device, a laser cavity having a resonant wavelength within the predetermined wavelength range and having an optical length such that a period of the RF modulation substantially equals a round-trip time for one of the pulses in the cavity, whereby the coherent optical pulses are output through the optical output assembly at a repetition rate substantially equal to the RF modulation.
Preferably, the semiconductor device includes a gain electrode for application of a current to the gain section.
Further preferably, the current includes a substantially DC current.
Preferably, the semiconductor device includes a common electrode which acts as a return for the gain electrode and the SA electrode.
Preferably, the semiconductor device includes a highly reflecting facet which together with the output facet encloses the optically-active region.
Further preferably, the output facet is coated by an antireflection coating.
Preferably, the optical output assembly includes a fiber optic having a fiber Bragg grating (FBG) which partially reflects the optical pulses within the predetermined wavelength range responsive to a period of the FBG, and wherein the fiber optic transmits the optical pulses.
Further preferably, the optical output assembly includes one or more lenses which focus the coherent optical pulses between the fiber optic and the output facet.
Preferably, the one or more lenses include a lens fixedly coupled to the fiber optic.
Further preferably, at least one of the one or more lenses and the fiber optic are positioned so as to form the laser cavity.
Preferably, the generator includes a DC bias current which is applied to the SA electrode.
Preferably, the gain section is positioned adjacent to the output facet.
Preferably, a length of the SA section is substantially less than a length of the gain section.
Preferably, the semiconductor device includes a passive waveguide section coupled to the optically-active region so as to form the laser cavity.
Preferably, the semiconductor device includes a highly reflecting facet which together with the output facet encloses the optically-active region and the passive waveguide section, and wherein the SA region is positioned adjacent to the output facet, so that a first optical length from the SA section to the highly reflecting facet is substantially equal to half a second optical length of the laser cavity.
There is further provide, according to a preferred embodiment of the present invention, a method for generating an optical pulse, including:
applying radio-frequency (RF) modulation of a predetermined frequency to a saturable absorber (SA) section of an optically-active region in a semiconductor device, the optically-active region comprising a gain section separate from the SA section, so as to generate coherent optical pulses at a repetition rate substantially equal to the predetermined frequency; and
coupling an optical output assembly to the optically-active region, so as to form a laser cavity that includes the optically-active region and has a resonant wavelength range substantially defined by the optical output assembly, and such that a period of the repetition rate substantially equals a round-trip time for one of the pulses in the cavity.
Preferably the method includes providing a gain electrode for application of a current to the gain section and an SA electrode for application of the RF modulation to the SA section and a common electrode which acts as a return for the gain electrode and the SA electrode.
Further preferably, the method includes enclosing the semiconductor device by a highly reflecting facet and an antireflection (AR) coated output facet, and wherein coupling the optical assembly to the optically-active region includes coupling the assembly via the AR coated facet.
Preferably, the optical output assembly includes a fiber optic having a fiber Bragg grating (FBG), and the method includes partially reflecting the optical pulses within the resonant wavelength range responsive to a period of the FBG.
Further preferably, the optical output assembly includes one or more lenses, and coupling the optical output assembly includes positioning at least one of the one or more lenses and the fiber optic so as to form the laser cavity.
Preferably, the method includes applying a DC bias current to the SA section.
Preferably, the method includes coupling a passive waveguide section to the optically-active region so as to form the laser cavity.
Further preferably, the method includes positioning the SA section substantially at an optical center of the laser cavity.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: